HOW POST-INSTALLED ANCHOR WORKS UNDER DIFFERENT LOADING CONDITIONS?

Fig. 1.1: Typical application in a building under construction

Fastening systems transfer applied loads to the base material in different ways. Under both tension (Fig. 2.1 a)) and shear loading (Fig. 2.1 b)), the load transfer mechanism involves the utilization of concrete tensile strength. We refer in this case to fastening design theory in opposition to the reinforced concrete theory, where the concrete tensile strength is usually neglected in design. The load-transfer mechanisms for various fastening systems are typically identified as mechanical interlock, friction, and adhesive bond mechanisms.

Fig. 2.1: Illustration of tensile capacity of concrete being utilized for load transfer by post-installed anchors (fastening design theory)

Post-installed anchors follow one or more of the mechanisms described in this section:

Mechanical interlock/keying defines the working principle where the load is transferred by means of a bearing surface between the anchor and the base material (see Fig. 2.2 a)). Some post-installed fasteners develop a mechanical interlock between the anchor and the base material. To achieve this, a cylindrically drilled hole is modified to create a notch, or undercut, of a specific dimension at a defined location either by means of a special drill bit, or by the undercutting action of the anchor itself. An example is the Hilti HDA.

Friction mechanism is the load-transfer mechanism typical of systems where expansion force is generated by a clip or a wedge pressed against the walls of the borehole during the installation process. Frictional resistance equilibrates the external tension force on anchors. The tensile load, N, is transferred to the base material by friction, R (Fig. 2.2 b)). An example is the Hilti HST4.

Adhesive bond mechanism involves the transfer of the external load to the concrete base material via an adhesive bond (see Fig. 2.2 c)). The forces are transferred from the anchor element (e.g., a threaded rod) to the mortar via mechanical interlocking and to the base material via a combination of micro-interlock and chemical adhesion between the mortar and the lateral surface of the borehole. An example is the Hilti HIT-RE 500 V4 & HAS Threaded Rods.

Fig. 2.2: Different types of load-bearing mechanisms in fastening technology and example of Hilti anchors following the mechanisms

Anchors can fail in various manner if the acting load exceeds their resistance. The failure modes can be distinguished for different loading directions, tension (Fig. 3.1) and shear (Fig. 4.1). Failure modes can further be distinguished between the rupture of the fasteners (steel failure) and the failure of the base material or of the interface between the anchor and base material (concrete failure).

Fig. 3.1: Different types of failures due to tension loading

Different steel and concrete related failure modes are described below.

Steel failure occurs when tension stresses induced by the acting load in the smallest cross section of the anchor exceed the ultimate steel resistance (Fig. 3.2).

Fig. 3.2: Steel failure under tension loading

To obtain a higher resistance to this failure mode, one of these strategies (or a combination of them) can be followed: 1) increase the number of anchors; 2) select a higher steel strength for the anchor or 3) increase the anchor diameter.

Concrete cone failure is characterized by the formation of a cone-shaped fracture surface originating in the load-transfer zone of the anchor and radiating towards the concrete surface with an angle of approx. 35° between the inclined radial crack and concrete surface (Fig. 3.3). The failure mode is also referred as concrete break-out under tension loading.

Fig. 3.3: Concrete cone failure under tension loading

To obtain a higher concrete cone resistance, one of the strategies (or a combination of them) can be followed: 1) increasing the spacing between anchors; 2) increasing the embedment depth of anchors; 3) using a base material of higher concrete strength class.

Pull-out failure occurs when the entire anchor is pulled out of the drilled hole without significant damage of the base material (Fig. 3.4).

Fig. 3.4: Pull-out failure in tension

To improve the pull-out resistance one of the strategies (or a combination of them) can be followed: 1) choice of an anchor with higher resistance; 2) increasing the anchor diameter; 3) increasing number of anchors.

Combined pull-out and concrete cone failure is applicable to bonded anchors only. This failure is a combination of the pull-out due to loss of bond between the anchor and the concrete and as a shallow concrete cone close to the concrete surface (Fig. 3.5).

Fig. 3.5: Combined pull-out and concrete cone failure under tension loading

Concrete splitting failure is caused by the hoop stresses around the anchor which originate from local load transfer and expansion forces that exceed the concrete tensile resistance (Fig. 3.6). This failure mode can occur during the installation of an anchor if the minimum spacing, edge distances or member thicknesses are not kept or due to loading in near edge/close to spacing conditions.

Splitting failure during installation can be avoided by maintaining the following conditions as given in the relevant ETA: 1) minimum edge distance 2) minimum spacing between anchors 3) minimum base material thickness.

Effective strategies to increase the resistance against splitting failure due to loading are: 1) increasing edge distance and spacing between fasteners; 2) reducing the embedment depth; and 3) accepting that splitting cracks will happen and re-run the design assuming cracked concrete and accounting for sufficient reinforcement in the base material to limit their width.

Fig. 3.6: Splitting failure under tension loading

Concrete blow-out failure is a result of high-bearing pressure generated in the load transfer area of the anchor (Fig. 3.7). These high-bearing stresses cause bursting forces transverse to the load direction, creating a concrete break-out on the side face of the member. This failure mode may be decisive in near edge conditions and large embedment that can usually be achieved with headed studs, but usually not with post-installed anchors. To avoid this failure, edge distance needs to be increased.

Fig. 3.7: Blow-out failure under tension loading

Post-installed anchors can experience steel or concrete related failures under shear loading with or without lever arm.

Fig. 4.1: Different types of failures due to shear loading

Steel failure occurs when tension stresses induced by the acting load in the smallest cross section of the anchor exceed the ultimate steel resistance (Fig. 4.2). If the shear load is applied with a lever arm the resistance is reduced due to the additional tension stress arising from the caused bending moment.

a) Failure without lever arm

b) Failure with lever arm

Fig. 4.2: Steel failure under shear loading

To increase the resistance against this failure mode: 1) select a more resistant steel material; 2) increase the diameter of the anchor; 3) increase the number of anchors.

Concrete pry-out failure primarily occurs in cases of limited embedment depth of anchors. It is caused by rotation of the fastener and the catenary tension force generated in the anchor bolt as a result of lateral deformation and the eccentricity between the acting shear force and the resultant resisting force in the concrete (Fig. 4.3). Pry-out failure is dependent on the resistance value for cone break-out and pull-out failure. Hence, if resistance for those failure modes can be increased, resistance against pry-out will also be higher.

Fig. 4.3: Concrete pry-out failure under shear loading

Concrete edge failure occurs under shear load when the anchors are close to an edge in the loading direction. It is characterized by the formation of a cone shaped fracture surface originating at the anchor shaft and radiating towards the concrete edge with an angle of approx. 35° (Fig. 4.4). This failure mode is also referred as concrete break-out under shear loading. The resistance against this failure mode can be improved by increasing: 1) the edge distance for first row of anchors; 2) the embedment depth of anchors; 3) the spacing between anchors in a group; and 4) diameter of anchors.

Fig. 4.4: Concrete Edge failure under shear loading